1. Field of the Invention
The present invention relates to a composition for dental restorative material and, more specifically, to a dental composition which is as excellent in surface smoothness as natural teeth, and less shrinkable by polymerization so that no gap can occur between the tooth and the restorative material, having excellent physical properties and being improved in terms of roentgenographic properties.
2. Prior Art
The filler used so far for dental composite resin restorative material is glass powders having a maximum particle diameter of 10 to 50.mu.m. However , the composite resin incorporating such a large particle-diameter filler fails clinically to give any smoothly finished surface comparable to natural teeth. Now, many makers present dental composite in which fillers resins fine-particle (hereinafter referred to as the fine fillers) having a mean particle diameter of 0.01 to 0.04 .mu.m only are incorporated to obtain surface smoothness.
However, these fine particle fillers have a large specific surface area and give rise in an increase in the amount of the monomers in the composite resins; that is, their proportion in the composite resins is as small as 30 to 60% by weight, making the physical properties likely to deteriorate, e.g., the shrinkage by polymerization likely to become large. There is also commercially available a certain type of composite resins which, as means for making up for this shrinkage by polymerization, uses an organic/inorganic composite filler obtained by mixing the monomer with a fine filler having a mean particle diameter of 0.01 to 0.04.mu.m, and curing and pulverizing the mixture. The organic/inorganic composite filler, because of having a small specific surface area, enables the amount of the monomer to be reduced, serves well to make up for the shrinkage by polymerization, and has surface smoothness as well. However, since the fine filler having a mean particle diameter of 0.01 to 0.04 .mu.m is used in the organic/inorganic composite filler, the content of the inorganic material in the cured product is very low. This in turn causes an increase in the coefficient of thermal expansion, which is otherwise responsible for lack of border seal in the mouth. In general, colloidal silica is used as the fine filler of the organic/inorganic composite filler, but this has a serious demerit of being lacking in the radioopacity that conservation material is required to have . Currently, restorative material using glass powders having a mean particle diameter of 2 .mu.m is mainly used and put on the market in the West. This restorative material has clinically sufficient surface smoothness. Still, this contains large amounts of monomer components and so poses a grave problem in connection with the shrinkage by polymerization.
We have made intensive and extensive studies as to a composition for dental restorative material which is as excellent in surface smoothness as natural teeth, is less shrinkable by polymerization so that no gap can be presented between the tooth and the restorative material, has excellent physical properties and is improved in terms of roentgenographic properties, and have achieved the present invention by finding that the object mentioned above is attained by the provision of a composition for dental restorative materials which comprises a novel combination of an organic/inorganic composite filler, a glass powder component and a fine filler.
We have also found that clinically sufficient surface smoothness is obtained by using a glass powder component having a maximum particle diameter of 10 .mu.m or less and a mean particle diameter of 0.1 to 5 .mu.m as a filler having roentgenographic properties. Additionally, we have found that if the roentgenographic glass powder component has such a particle diameter, then the glass powder component accounts for 60% or more by weight of the organic/inorganic composite filler, and that the combination of the glass powder component with the organic/inorganic composite filler provides a restorative material which has a reduced coefficient of thermal expansion, is less shrinkable by polymerization and is improved in terms of physical properties as well. The incorporation of the transparent, roentgenographic glass powder component in the filler achieves clinically sufficient roentgenographic properties and imparts sufficient transparency to the restorative material.